The AU-UV beam line on ASTRID2

Photoabsorption measurements.

High resolution photoabsorption spectra of almost 200 molecules have been measured on the UV beam line. Using the high energy grating (HEG) and entrance/exit slits set to 0.1 mm the photon resolution is ~0.08 nm, allowing fine structure due to, for example, Rydberg states to be resolved.

Photoabsorption spectra of chloromethane, iodomethane and trifluoro-iodomethane recorded on the the UV beam line.

The apparatus used for measurement of gas phase photoabsorption spectra has undergone several alterations since that originally built in 2000 for use on UV1 (described in Eden 2006 - see publications list). The cell, which allows moderate (up to 80oC) heating of a sample during measurement, has a path length of 15.5 cm and is fitted with a heated 1 Torr baratron capacitance manometer (Setra model 774). The light exits the cell through a MgF2 window, which sets the lower limit of detectable light to 115 nm. A small gap between the PMT detector and the absorption cell is evacuated using a scroll pump for measurements below 200 nm. Above 200 nm air is allowed into this gap to let oxygen absorb higher orders of light (at half the chosen wavelength) which may be passing through the cell. In this way, photoabsorption measurements can be performed with spectrally pure radiation from 115 nm to 340 nm, ensuring artefact free spectra.

Schematic diagram of the gas phase photoabsorption apparatus. The main body of the cell and the sample can be heated, while keeping the detector and pumps at room temperature. The baratron is separately heated to 123oC to avoid molecular condensation.

Absolute photoabsorption cross-sections (σ) are obtained using the Beer-Lambert law

It = I0 exp(-nσl)

where It and I0 are the detector signal transmitted through a gas and for an evacuated cell, respectively. l is the cell pathlength (15.5 cm) and n is the molecular number density (derived from the pressure measument). These quantities are all wavelength dependent. ASTRID2 is operated in a so-called top-up mode, keeping the stored electron beam current (and thus the intensity for a given wavelength) quasi constant by adding small amounts of current to ASTRID2 to make up for the constant beam decay. The beam current thus varies about 3-5% during a scan, and this is taken into account by recording an accurately determined ring current.